As mobile device technology continues to develop and demand therefor continues to increase, demand for secondary batteries as energy sources is rapidly increasing. Among these secondary batteries, lithium secondary batteries, which exhibit high energy density and operating potential, have long cycle lifespan, and have a low self-discharge rate, are commercially available and widely used.
In addition, as interest in environmental problems is increasing, research into electric vehicles and hybrid electric vehicles that can replace vehicles using fossil fuels, such as gasoline vehicles, diesel vehicles, and the like, which are one of the main causes of air pollution, is actively underway. As a power source of electric vehicles, hybrid electric vehicles, and the like, a nickel metal-hydride secondary battery is mainly used. However, research into lithium secondary batteries having high energy density and high discharge voltage is actively underway and some lithium secondary batteries are commercially available.
In a lithium ion secondary battery used in conventional small batteries, generally, a cathode is formed of a lithium cobalt composite oxide having a layered structure and an anode is formed of a graphite-based material. However, a lithium cobalt composite oxide is unsuitable for use in batteries for electric vehicles because Co, which is a main component of the lithium cobalt composite oxide, is very expensive and the lithium cobalt composite oxide is not safe. Thus, lithium manganese composite oxides, which are inexpensive, have high safety, consist of Mn, and have a spinel structure, may be suitable for use in a cathode of lithium ion batteries for electric vehicles.
In general, spinel-structure lithium manganese-based oxides have high thermal safety, are inexpensive, and are easy to synthesize, while having low capacity, deteriorated lifespan characteristics due to side reaction, poor high-temperature characteristics, and low conductivity.
To address these problems, use of lithium manganese composite oxides having a spinel structure, some metal elements of which are substituted, has been tried. For example, Korean Patent Application Publication No. 2002-65191 discloses a spinel-structured lithium manganese composite oxide with high thermal safety. However, a battery including the lithium manganese composite oxide exhibits low capacity and deteriorated high-temperature storage characteristics and cycle lifespan.
To complement the low capacity problems of spinel and secure excellent thermal stability of manganese-based active materials, use of lithium manganese composite oxides having a layered structure has been tried. However, such lithium manganese composite oxides have an unstable structure, undergo phase transition during charge and discharge, and exhibit rapidly reduced capacity and deteriorated lifespan characteristics.
In addition, when such lithium manganese composite oxides are stored at high temperature, Mn is eluted to an electrolyte by the impact of the electrolyte and thus battery characteristics are deteriorated and therefore there is a need to develop improvements to address these problems. In addition, such lithium manganese composite oxides have lower capacity per unit weight than existing lithium cobalt composite oxides or conventional lithium nickel composite oxides and thus there are limitations in increasing capacity per unit battery weight. Thus, batteries that address these problems need to be designed to enable practical use thereof as a power source of electric vehicles.
Therefore, there is an urgent need to develop technology for enhancing structural stability of a cathode active material at high voltage without deterioration of battery characteristics.